A linear alpha-olefin, which is an important material used for a comonomer, a cleaning agent, a lubricant, a plasticizer, and the like, is commercially widely used, and particularly, 1-hexene and 1-octene are frequently used as a comonomer for controlling the density of polyethylene when preparing a linear low-density polyethylene (LLDPE) and the like.
More specifically, in the existing preparation process of LLDPE (linear low-density polyethylene), ethylene is copolymerized with alpha-olefin comonomers such as 1-hexene and 1-octene, so as to form a branch in a polymer backbone to control the density thereof.
Thus, there is a problem in that the cost of comonomers accounts for a large part of the production cost in the preparation of LLPDE having a high comonomer content. Accordingly, various attempts have been made to reduce the production cost of the comonomers.
Also, since alpha-olefins have a different application field or market size according to the kind, a technology for selectively producing specific alpha-olefins is commercially very important, and recently, many studies have been carried out on a chromium catalyst technology for preparing 1-hexene or 1-octene with high selectivity through selective ethylene oligomerization.
The existing commercial methods for preparing 1-hexene or 1-octene include a SHOP process of Shell Chemicals, a Ziegler process of Chevron Philips, and the like, whereby C4-C20 alpha-olefins with wide distributions can be produced.
As a catalyst used in the production of selective alpha-olefins, for example, as an existing catalyst for trimerization of ethylene, a chromium-based catalyst using a ligand of a General Formula (R1)(R2)X—Y—X(R3)(R4) has been suggested. In the Formula, X is phosphorous, arsenic, or antimony, Y is a linking group such as —N(R5)-, and at least one of R1, R2, R3, and R4 has a polar or electron donating substituent.
In addition, as a ligand that does not exhibit catalytic activity to 1-hexene under a catalytic condition, studies have been carried out on (o-ethylphenyl)2PN(Me)P(o-ethylphenyl)2, a compound which does not have a polar substituent on at least one of R1, R2, R3, and R4 (Chem. Commun., 2002, 858).
However, the existing ligand containing a heteroatom as described above, and the chromium-based catalyst obtained therefrom, are difficult to consistently and excellently exhibit multimerization activity, for example, during the production of 1-octene or 1-hexene, and the selectivity thereof is also not sufficient. Thus, there is continued demand for improving them.